How to hack TinyScheme ---------------------- TinyScheme is easy to learn and modify. It is structured like a meta-interpreter, only it is written in C. All data are Scheme objects, which facilitates both understanding/modifying the code and reifying the interpreter workings. In place of a dry description, we will pace through the addition of a useful new datatype: garbage-collected memory blocks. The interface will be: (make-block []) makes a new block of the specified size optionally filling it with a specified byte (block? ) (block-length ) (block-ref ) retrieves byte at location (block-set! ) modifies byte at location In the sequel, lines that begin with '>' denote lines to add to the code. Lines that begin with '|' are just citations of existing code. Lines that begin with X are deleted. First of all, we need to assign a typeid to our new type. Typeids in TinyScheme are small integers declared in an enum, very close to the top of scheme.c; it begins with T_STRING. Add a new one before the end, call it T_MEMBLOCK. Adjust T_LAST_SYSTEM_TYPE. | T_ENVIRONMENT=14, X T_LAST_SYSTEM_TYPE=14 > T_MEMBLOCK=15, > T_LAST_SYSTEM_TYPE=15 | }; Then, some helper macros would be useful. Go to where is_string() and the rest are defined and define: > int is_memblock(pointer p) { return (type(p)==T_MEMBLOCK); } This actually is a function, because it is meant to be exported by scheme.h. If no foreign function will ever manipulate a memory block, you can instead define it as a macro > #define is_memblock(p) (type(p)==T_MEMBLOCK) Then we make space for the new type in the main data structure: struct cell. As it happens, the _string part of the union _object (that is used to hold character strings) has two fields that suit us: | struct { | char *_svalue; | int _keynum; | } _string; We can use _svalue to hold the actual pointer and _keynum to hold its length. If we couldn't reuse existing fields, we could always add other alternatives in union _object. We then proceed to write the function that actually makes a new block. For conformance reasons, we name it mk_memblock > static pointer mk_memblock(scheme *sc, int len, char fill) { > pointer x; > char *p=(char*)sc->malloc(len); > > if(p==0) { > return sc->NIL; > } > x = get_cell(sc, sc->NIL, sc->NIL); > > typeflag(x) = T_MEMBLOCK|T_ATOM; > strvalue(x)=p; > keynum(x)=len; > memset(p,fill,len); > return (x); > } The memory used by the MEMBLOCK will have to be freed when the cell is reclaimed during garbage collection. There is a placeholder for that staff, function finalize_cell(), currently handling strings only. | static void finalize_cell(scheme *sc, pointer a) { | if(is_string(a)) { | sc->free(strvalue(a)); > else if(is_memblock(a)) { > sc->free(strvalue(a)); | } else if(is_port(a)) { There are no MEMBLOCK literals, so we don't concern ourselves with the READER part (yet!). We must cater to the PRINTER, though. We add one case more in atom2str(). | } else if (is_foreign(l)) { | p = sc->strbuff; | snprintf(p,STRBUFFSIZE,"#", procnum(l)); > } else if (ismemblock(l)) { > p = "#"; | } else if (is_continuation(l)) { | p = "#"; | } else { Whenever a MEMBLOCK is displayed, it will look like that. Now, we must add the interface functions: constructor, predicate, accessor, modifier. We must in fact create new op-codes for the virtual machine underlying TinyScheme. Since version 1.30, TinyScheme uses macros and a single source text to keep the enums and the dispatch table in sync. That's where the op-codes are declared. Note that the opdefines.h file uses unusually long lines to accommodate all the information; adjust your editor to handle this. The file has six columns: A to Z. they contain: - Column A is the name of a routine to handle the scheme function. - Column B is the name the scheme function. - Columns C and D are the minimum and maximum number of arguments that are accepted by the scheme function. - Column E is a set of flags that are used when the interpreter verifies that the passed parameters are of the correct type. - Column F is used to create a set of enums. The enum is used in a switch in the routine listed in column A to get to the code that does the work needed for the scheme function. For reasons of cohesion, we add the new op-codes right after those for vectors: | _OP_DEF(opexe_2, "vector-set!", 3, 3, TST_VECTOR TST_NATURAL TST_ANY, OP_VECSET ) > _OP_DEF(opexe_2, "make-block", 1, 2, TST_NATURAL TST_CHAR, OP_MKBLOCK ) > _OP_DEF(opexe_2, "block-length", 1, 1, T_MEMBLOCK, OP_BLOCKLEN ) > _OP_DEF(opexe_2, "block-ref", 2, 2, T_MEMBLOCK TST_NATURAL, OP_BLOCKREF ) > _OP_DEF(opexe_2, "block-set!", 1, 1, T_MEMBLOCK TST_NATURAL TST_CHAR, OP_BLOCKSET ) | _OP_DEF(opexe_3, "not", 1, 1, TST_NONE, OP_NOT ) We add the predicate along the other predicates: | _OP_DEF(opexe_3, "vector?", 1, 1, TST_ANY, OP_VECTORP ) > _OP_DEF(opexe_3, "block?", 1, 1, TST_ANY, OP_BLOCKP ) | _OP_DEF(opexe_3, "eq?", 2, 2, TST_ANY, OP_EQ ) All that remains is to write the actual processing in opexe_2, right after OP_VECSET. > case OP_MKBLOCK: { /* make-block */ > int fill=0; > int len; > > if(!isnumber(car(sc->args))) { > Error_1(sc,"make-block: not a number:",car(sc->args)); > } > len=ivalue(car(sc->args)); > if(len<=0) { > Error_1(sc,"make-block: not positive:",car(sc->args)); > } > > if(cdr(sc->args)!=sc->NIL) { > if(!isnumber(cadr(sc->args)) || ivalue(cadr(sc->args))<0) { > Error_1(sc,"make-block: not a positive number:",cadr(sc->args)); > } > fill=charvalue(cadr(sc->args))%255; > } > s_return(sc,mk_memblock(sc,len,(char)fill)); > } > > case OP_BLOCKLEN: /* block-length */ > if(!ismemblock(car(sc->args))) { > Error_1(sc,"block-length: not a memory block:",car(sc->args)); > } > s_return(sc,mk_integer(sc,keynum(car(sc->args)))); > > case OP_BLOCKREF: { /* block-ref */ > char *str; > int index; > > if(!ismemblock(car(sc->args))) { > Error_1(sc,"block-ref: not a memory block:",car(sc->args)); > } > str=strvalue(car(sc->args)); > > if(cdr(sc->args)==sc->NIL) { > Error_0(sc,"block-ref: needs two arguments"); > } > if(!isnumber(cadr(sc->args))) { > Error_1(sc,"block-ref: not a number:",cadr(sc->args)); > } > index=ivalue(cadr(sc->args)); > > if(index<0 || index>=keynum(car(sc->args))) { > Error_1(sc,"block-ref: out of bounds:",cadr(sc->args)); > } > > s_return(sc,mk_integer(sc,str[index])); > } > > case OP_BLOCKSET: { /* block-set! */ > char *str; > int index; > int c; > > if(!ismemblock(car(sc->args))) { > Error_1(sc,"block-set!: not a memory block:",car(sc->args)); > } > if(isimmutable(car(sc->args))) { > Error_1(sc,"block-set!: unable to alter immutable memory block:",car(sc->args)); > } > str=strvalue(car(sc->args)); > > if(cdr(sc->args)==sc->NIL) { > Error_0(sc,"block-set!: needs three arguments"); > } > if(!isnumber(cadr(sc->args))) { > Error_1(sc,"block-set!: not a number:",cadr(sc->args)); > } > index=ivalue(cadr(sc->args)); > if(index<0 || index>=keynum(car(sc->args))) { > Error_1(sc,"block-set!: out of bounds:",cadr(sc->args)); > } > > if(cddr(sc->args)==sc->NIL) { > Error_0(sc,"block-set!: needs three arguments"); > } > if(!isinteger(caddr(sc->args))) { > Error_1(sc,"block-set!: not an integer:",caddr(sc->args)); > } > c=ivalue(caddr(sc->args))%255; > > str[index]=(char)c; > s_return(sc,car(sc->args)); > } Same for the predicate in opexe_3. | case OP_VECTORP: /* vector? */ | s_retbool(isvector(car(sc->args))); > case OP_BLOCKP: /* block? */ > s_retbool(ismemblock(car(sc->args)));